In recent years, internal combustion engines have been required to attain fuel savings, and engine oil having lower viscosity is employed in order to satisfy the requirement. Conventionally, viscosity of engine oil has been lowered through appropriately blending high-viscosity oil with low-viscosity oil. Low-viscosity oil readily evaporates at high temperature in an engine, and is gradually discharged with exhaust gas. Thus, such an engine oil comes to have higher viscosity, thereby impairing fuel economy. Meanwhile, in order to assess such a drawback of conventional engine oil, the Noack test has been employed for providing an index for volatility. As a result, a lube base oil satisfying low volatility has been attained and come to be used more frequently.
Ester-based oil compositions are known to be lubricating oil compositions which have low viscosity and exhibit low evaporation loss. However, these compositions have high polarity and have adverse effects on organic materials, such as inducing swelling. For example, when such an ester-based oil composition is used in an engine oil, problematic swelling of a sealing material and-other materials made of rubber occurs. When the composition is used in an oil for hydraulic bearings or oilless bearings, plastic materials are adversely affected. In addition, these ester-based oil compositions have poor solubility with respect to additives, which is also problematic.
Silicone oil serves as a lube base oil which has low viscosity and exhibits low evaporation loss, but has problems of poor lubricity and high cost. Poly(α-olefin) is known to be a base oil which has low viscosity and exhibits low evaporation loss. However, a poly(α-olefin) categorized in a viscosity grade of 4 mm2/s (100° C.) is unsatisfactory in terms of low viscosity and a poly(α-olefin) categorized in a viscosity grade of 2 mm2/s (100° C.) is unsatisfactory in terms of low volatility.
Japanese Patent Application Laid-Open (kokai) No. 10-324883 discloses a lube base oil which contain a compound having an ether bond and exhibits a viscosity index and a pour point falling within a limited range. However, in order to satisfy further fuel cost saving performance, a lube base oil having lower viscosity and exhibiting small evaporation loss is demanded.